1. Field of the Invention
This invention pertains to connection configurations for tubular products and more particularly to augmenting the metal-to-metal sealing of the connections of such products and protecting the surfaces involved in such sealing connections.
2. Description of the Prior Art
Tubular goods generally comprise a plurality of individual joints connected together pin end to box end. The pin end of a tubular joint is the male end and generally includes at least one set of helical or spiral external threads. The box end is the accommodating female end and generally includes mating internal helical or spiral threads.
Tubular goods employed in the oil and gas industry include both the casing string and the production tubing string. In both cases the environmental operating conditions are often at high pressures and at high temperatures. The most effective and certain seals used in the industry today to prevent leaks in the connections of tubular goods are metal-to-metal seals. Such seals have extremely favorable sealing performance characteristics compared to rubber or other seals employing non-metal materials.
To effect a metal-to-metal seal, the pin end of the joint incorporates a sealing surface forward of the thread set or leading thread set on the pin end. This surface generally is tapered for establishing a conical sealing surface. The accommodating box end is similarly provided with a receiving shoulder sealing surface deep within the well of the box end and just beyond the limits of its thread set or deepest thread set. If the taper of the metal sealing surface of the pin end is at the preferred angle of 14.degree. then this box end sealing surface is at the same angle. Therefore, when the pin end is screwed into the box end, the leading pin thread completely passes through the box end threads until sealing surface contact is made of the pin end with the box end. A further tightening of the connection after contact causes conical deformation of either or both the pin end and box end at the contact location. When the connection is completely made up there is metal-to-metal sealing surface engagement adjacent the threads and a slight wedge separation on the non-thread side of such engagement.
The engagement of the metal surfaces is perfectly satisfactory provided the surfaces are free of blemishes or foreign matter. However, tubular goods of the type just described are not uncommonly subjected to fairly rough treatment. For example, when two joints are brought together in a stabbing operation preliminary to connection makeup, the end of the pin is often scratched or gouged in a stabbing attempt with the ends slightly misaligned. In fact, an attempt may even be made to rotate a misaligned connection in hope and anticipation that the connection is aligned properly for makeup, not realizing that the joints are misaligned. Such rotation may cause grooving of the pin end sealing surface. Furthermore, tubular joints are often dragged in handling so that their ends are scraped. Again, this can result in damage to the pin end sealing surface. When the joints are then made up in the previously described manner, there is a weakness in the metal-to-metal seal engagement with the existence of one or more paths through which pressure leaks can occur.
The problem of scarring the pin end sealing surface in one or more of the manners just described or otherwise has long been recognized. It has been believed heretofore that when the sealing surface was damaged sufficiently to prevent effecting a satisfactory seal, the joint had to be taken out of service. If the surface was not too badly damaged, it could be remachined. If it was damaged a little, then either the connection made with the slightly damaged pin end had to be tested, which is a cumbersome operation at best under field conditions, or the decision had to be made, without adequate data, to use the joint as is or to remove it from service. A decision to remove an acceptacle joint is closely. A decision to use an unacceptable joint can be disastrous. Moreover, a perfectly good joint could be damaged in a makeup attempt just prior to makeup, also with disasterous consequences.
Many connections include an auxiliary or back-up or duplicate sealing area to minimize the possibility of failure when there is a bad metal-to-metal seal because of a leakage path condition, as just described. Such connections provide sealing surfaces at other locations in the thread configuration in addition to the metal-to-metal seal location described above.
Attempts to use a seal utilizing a material other than metal have not proved satisfactory because of their inability to satisfactorily operate as intended under the extreme pressure and temperature conditions that exist for the larger tubular products, for example, tubular products from 27/8" to 14" used in conditions where the pressure is in excess of 20,000 psig and the temperature is up to about 450.degree. F. An example of an unsatisfactory structure under these conditions is shown in U.S. Pat. No. 4,253,687 entitled "Pipe Connection", issued Mar. 3, 1981. This patent describes grooving the tapered surface of the pin end of a pipe connection and the placement therein of an annular O-ring made of Teflon or similar material. This would be done when the sealing surface was such that it would no longer be able to makeup a metal-to-metal seal. The O-ring would extend radially from the groove (FIG. 7) so that when the connection was subsequently made up (FIG. 8) the material would seal against the box surface (35) and even extrude slightly up the annulus between the pin and the box (at 77).
Although possibly acceptable for small tubular products and for low pressure conditions, such as arrangement is totally unsuited for larger tubular goods and for higher pressure conditions. Under high pressures downhole, for example, the O-ring would not only extrude slightly, as shown in FIG. 7 of the '687 patent, it would be blown up annulus 75 and cause complete sealing failure.
Therefore, it is a feature of the present invention to provide an improved tubular connection for causing a compound seal with a conical metal-to-metal seal, the improved connection including an elastomeric material conforming to the metal-to-metal seal and even to minor blemishes therein.
It is another feature of the present invention to provide an improved compound sealing tubular connection employing an elastomeric compound that is used in such a way that it can be employed even in high pressure environmental conditions.
It is still another feature of the present invention to provide an improved tubular connection having an elastomeric material component that operates in making a compound seal, protects the end of the joint of which is is a part from damage, and is replaceable when the need arises.